Positron emission tomography (PET) is a nuclear medicine imaging technique that produces a three-dimensional image representing the distribution of positron emitting isotopes within a body. As the radioisotope undergoes positron emission decay (also known as positive beta decay), it emits the antimatter counterpart of electrons. As the positrons lose energy, they ultimately encounter and annihilate with electrons, usually producing a pair of annihilation (gamma) photons moving in opposite directions. The PET system determines the line along which the annihilation occurred, by detecting the pair of gamma photons in time coincidence.
In general, a test object (phantom) is used to calibrate and/or verify (QC) the accuracy of nuclear medical imaging devices such as PET scanners. In essence, a phantom is an object that contains positron (β+) emitting activity in a known shape and distribution throughout its body. Thus, by imaging the phantom, the accuracy of the PET instrument and its software may be assessed and, if necessary, the settings may be adjusted.
Conventionally, PET, PET/CT, and PET/MR systems are quality controlled and calibrated using a cylindrical positron source, also referred to as a phantom, comprised of 68Ge, as the positron source, dispersed in a solid urethane matrix, and encased in a polyethylene shell. 68Ge decays to the positron-emitting isotope 68Ga. The cylindrical shape facilitates subsequent analysis through symmetry. The cylinder is longer than the axial extent of the imaging volume, so dimensions of 21 cm diameter×35 cm long are typical, and they are also quite heavy. As these phantoms are radioactive, they unavoidably expose the operators to some ionizing radiation when positioned within, and removed from, the imaging system. Scaling such phantoms to support systems with ever increasing axial extent is impractical as there is excessive attenuation and scatter of the gamma photons, and the phantoms become excessively heavy and difficult to shield when in storage.
The afore mentioned phantoms are heavy, about 10-15 kg, and require over 100 kg of lead for shielding the technologist, the scanner, patients and other people, from the bulk of the annihilation gamma photons when not in use. These weights impact cost of shipment and facility of handling. Further, the shielding contains lead and as such is a challenge from a hazardous material control perspective.
Thus a need exists for a phantom that can accommodate a PET scanner, a PET/CT scanner, and/or a MR/PET scanner, and overcomes the problems described in the prior art.